1. Field of the Invention
The invention pertains to the field of high voltage solid state switches. More particularly, the invention pertains to triggering a high voltage solid state switch comprised of a set of solid state devices connected in series.
2. Description of Related Art
Spark gap, krytron, thyratron, sprytron, and other high voltage switches are used in many military, research, medical, and commercial applications where switching of tens of kilovolts and thousands of amperes is required. It is desirable to replace these switches with solid state switches having similar operational characteristics but with longer lifetimes.
Because existing commercial solid state high power devices, such as insulated gate bipolar transistors (IGBTs), metal-oxide semiconductor field-effect transistors (MOSFETs) and thyristors, cannot operate at tens of kilovolts, multiple devices in series are required. In many cases a snubber circuit is also required for proper operation. A snubber circuit may be used to ensure equal voltage distribution between the multiple devices in series as the voltage across the switch changes. A capacitor in series with a resistor can serve as the snubber circuit.
Triggering the individual devices can be a complicated task, since each device is floating at a different voltage. One means of triggering such devices uses a large pulse transformer, with appropriate high voltage isolation between the primary and secondary windings and with high voltage isolation between the multiple secondary windings, in order to bring the low voltage trigger pulse to the devices. Because of the inefficiency, size and cost of the large pulse transformer, this method becomes less desirable as the switch voltage and number of series connected devices increases.
In response, methods were developed using power stored in a capacitor floating with the device for the trigger energy. These methods used low power triggers for a low power solid state device that discharged the capacitor into the gate of the high power device. While still requiring a pulse transformer, because of the lower energy requirements, it could be smaller. General examples of these switches can be seen in U.S. Pat. Nos. 5,444,610 and 5,646,833.
U.S. Pat. Nos. 6,396,672 and 6,710,994 describe triggering systems that use the energy stored in a snubber capacitor to provide the necessary energy to trigger a solid state device. U.S. Pat. No. 6,624,684 describes a compact method for triggering multiple solid state devices connected in series, again using the energy stored in a snubber capacitor to provide the energy to trigger the solid state device. With these, either a low power electrical signal through a pulse transformer or an optical signal can be used to trigger the switch.
U.S. Pat. Nos. 5,933,335 and 5,180,963 are examples of an optically triggered switch. In U.S. Pat. No. 5,180,963, there is an optical signal for each set of two high power solid state devices. The optical signal triggers a phototransistor which in turn triggers a low power solid state device. The low power solid state device then discharges a capacitor through a pulse transformer, producing signals in the gates of the two high power solid state devices that turn on the devices.
In all of these types of switches, multiple phototransistors, pulse transformers, low power solid state devices, and other components, are required, a set for each high power solid state device being triggered. These components cause the high voltage solid state switch to be larger and more expensive, on top of the already large price for the high voltage solid state devices.